1. Field of the Invention
The present invention is directed to low pressure mercury vapor lamps, more commonly known as fluorescent lamps, having a lamp envelope with phosphor coating, and more particularly, to a coating with three phosphors over an alumina pre-coat.
2. Discussion of the Prior Art
Low pressure mercury vapor lamps, more commonly known as fluorescent lamps, have a lamp envelope with a filling of mercury and rare gas to maintain a gas discharge during operation. The radiation emitted by the gas discharge is mostly in the ultraviolet (UV) region of the spectrum, with only a small portion in the visible spectrum. The inner surface of the lamp envelope has a luminescent coating, often a blend of phosphors, which emits visible light when impinged by the ultraviolet radiation. Special fluorescent lamps are used in horticulture and are referred to as xe2x80x9cAgroxe2x80x9d or xe2x80x9cAgro-Litexe2x80x9d lamps.
Agro lamps used in horticulture contain phosphors that simulate the photoperiod of daylight or of natural light. The Agro lamps are used for growing plants indoors and have been developed with phosphors that closely match the absorption spectra of chlorophyll. The phosphors of Agro lamps are rich in the blue and red regions of the spectrum since plants use blue light (approximately 450 nm) for root growth, and red light (approximately 600-700 nm) for photosynthesis, stem growth, flowering and chlorophyll production. The blue/violet light also inhibits bacteria and growth of molds.
The phosphors of conventional Agro lamps are high mercury consumers and cannot pass the Toxicity Characteristic Leaching Procedure (TCLP) test without sacrificing lamp life. Accordingly, there is a drive to reduce mercury consumption in Agro fluorescent lamps without a reduction in the lamp life.
To increase efficiency and reduce mercury consumption without a reduction in the lamp life, different blends of phosphors are used for the luminescent coating. Further, a metal oxide layer is provided between the luminescent coating and glass envelope. The metal oxide layer reflects the UV radiation back into the phosphor luminescent layer through which it has already passed for further conversion of the UV radiation to visible light. This improves phosphor utilization and enhances light output. The metal oxide layer also reduces mercury consumption by reducing mercury bound at the tubular portion of the lamp.
Desirable fluorescent lamps characteristics include high brightness and high color rendering. Conventional Agro lamps have a correlated color temperature of 2450 K, with a CRI greater than 80. In particular, a conventional Agro lamp is made with a two-phosphor mixture of Strontium Magnesium Phosphor (Sr. Mag), i.e., (Sr,Mg)3(PO4)2:Sn, and Strontium Chloroapatite (SCAP), i.e., Sr5Cl(PO4)3:Eu. The Sr. Mag is very rich in the red region of the spectrum and the SCAP provides the Agro lamp with the blue light source.
These phosphors are detrimental for mercury consumption. In particular, Sr. Mag is the highest consumer of mercury and its high percentage renders the conventional Agro lamps non-TCLP compliant.
Accordingly, there is a need for fluorescent Agro lamps with high CRI and reduced mercury that pass TCLP.
The object of the present invention is to provide fluorescent Agro lamps with high CRI and reduced mercury consumption.
The present invention accomplishes the above and other objects by providing an electric lamp having an envelope with an inner surface and at least one electrode, such as two electrodes located at both ends of the envelope tube. The electrodes transfer electric power to generate ultraviolet radiation in the envelope which is filled with mercury and a charge sustaining gas. The inner surface of the envelope is pre-coated with a metal oxide layer, such as an aluminum oxide layer, to reflect ultraviolet radiation back into the envelope.
A phosphor layer is formed over the aluminum oxide to convert the ultraviolet radiation to visible light. The phosphor layer is a mixture of three phosphors, namely, blue luminescing Barium Magnesium Aluminate (BAM), red-luminescing Cerium Gadolinium Magnesium Borate (CBTM), and 3000 K-luminescing Calcium Halophosphor, also referred to as Warm White (WW).